Recent years have seen advances in the introduction of communication systems directed toward reducing traffic accidents by using communication between road and vehicle (a road-side device and a target vehicle) or between vehicle and vehicle (a target vehicle and other vehicles) to report to the target vehicle that is traveling on a road support information for safe driving that indicates, for example, the state of roads in the vicinity or the state of movement of vehicles in the vicinity.
The support information for safe driving that is provided in such a communication system usually takes as targets vehicles that are present in a fixed direction and within a fixed distance from a transmission-origin vehicle that is the transmission origin of information (hereinbelow referred to as “transmitting vehicle”). For example, in one support system that provides information regarding emergency vehicles that is considered as a one vehicle-to-vehicle communication application, information is provided from an emergency vehicle that is the transmitting vehicle to vehicles that are present within a range of approximately 300 m ahead of the emergency vehicle.
Although the use of microwave electromagnetic waves is effective in this communication system, high-frequency electromagnetic waves such as microwaves also have the properties of short propagation distance and high diffraction loss. As a result, the transmitting vehicle is, in some cases, unable to directly communicate with vehicles present within the delivery target area (hereinbelow referred to as the “transmission destination area”).
FIG. 1 shows an example of a method for expanding the transmission destination area of the transmitting vehicle. As shown in FIG. 1, vehicle B that is capable of forwarding transmission information that is transmitted from transmitting vehicle A (hereinbelow referred to as “relay vehicle”) is able to expand the transmission destination area by successively forwarding the transmission information.
In this case, upon receiving transmission packets in which transmission information is included, relay vehicle B forwards the transmission packets outside the transmission destination area. By being repeatedly carried out by the relay vehicle, this wasteful relay not only increases the communication traffic but also impedes the appropriate delivery of information.
In contrast, a geocast routing method is known that designates the area to which transmission packets are to be forwarded. FIGS. 2 and 3 show example of the designated areas that are designated in this geocast routing method.
FIG. 2 is a conceptual view for a case in which the positional coordinates of four points are used to set a rectangular designated area by the geocast routing method. FIG. 3 is a conceptual view for a case in which a round designated area is set by means of the center coordinates and the radius by the geocast routing method.
As a radio network system that employs the geocast routing method, Patent Document 1 describes a radio network system that uses parameters that indicate the relay direction of packets and the effective width of the relay area in which packets are to be relayed to set a designated area.
In the radio network system described in Patent Document 1, a transmission origin terminal device designates parameters that indicate the relay direction and the effective width of the relay area, appends these parameters (hereinbelow referred to as “designation parameters”) to packets, and then transmits the packets. A relay terminal device, upon receiving these packets, judges whether the current position of its vehicle is within the designated area that is determined by the designation parameters. If the current position of its own vehicle is not contained in the designated area, the relay terminal device halts the relay of the packets. As a result, the radio network system is able to decrease wasteful relays by a relay vehicle and thus reduce the congestion of packet transmission.